The field of the invention relates to two-part urethane structural adhesives and bonding of structural components therewith such as in the construction, automotive and marine industries.
Two-component polyurethane adhesives have long been known. Thus, U.S. Pat. No. 3,274,160 describes the reaction products of polyisocyanates with a mixture containing alcoholic hydroxyl functions.
Polyurethanes obtained from polyester polyol prepolymers which are mixed with low molecular weight polyols and reacted with diisocyanates are described in U.S. Pat. No. 4,182,898.
A method of preparation of polyurethane adhesives using excess diisocyanates, polyols, and optionally chain lengthening agents such as amines is described in U.S. Pat. No. 4,156,064.
U.S. Pat. No. 4,487,909 describes an adhesive based on a polyol, e.g. a polyether polyol. and glycerol as branching agent.
U.S. Pat. No. 4,554,340 describes a polyol mixture for the preparation of polyurethanes from high molecular weight polyalkylene oxides and low molecular weight diols and optionally also isocyanate reactive compounds such as glycerol and diamine.
U.S. Pat. No. 3,714,127 describes two-component polyurethane adhesives based on an isocyanate prepolymer to which an amine is added for increasing the viscosity and ensuring the non-sagging character of the adhesive.
EP 0,068,209 describes a two-component polyurethane adhesive having a polyol component containing a polyester triol or polyether triol, a phenylene diamine, and a tin catalyst. This mixture is reacted with an aromatic diisocyanate.
In U.S. Pat. No. 4,554,340, there is disclosed a polyol mixture for use in the preparation of polyurethane. The polyol mixture comprises a high molecular weight polyalkylene oxide, low molecular weight diol, and optionally isocyanate-reactive compounds including glycerine and diamines. The mixture is particularly characterized in that component (a) is a polyoxyalkylene polyol having an OH-number of from 20 to 210 which contains at least 5% and less than 80%, by weight of terminal oxyethylene blocks, component (b) is a low molecular weight diol mixture having an OH-number of from 1200 to 1650 which has the form of a reaction product of 1 mole of ethylene glycol with from 0.1 to 0.5 moles of propylene oxide
U.S. Pat. No. 3,642,943 discloses a solution of polymeric material which contains a mixture of an isocyanate-containing urethane prepolymer and a copolymer of an acrylic monomer and an adduct of an organo-diisocyanate and a hydroxyl alkyl acrylic monomer. The isocyanate-containing urethane prepolymer is prepared by reacting an organo-diisocyanate and a polyol or a mixture of polyols. Various polyols described as being useful in forming the urethane prepolymer include polyether polyols prepared by the addition polymerization of ethylene oxide and/or propylene oxide and a polyol like trimethylol propane. Other polyols mentioned include polyester polyols prepared by copolymerizing a low molecular weight polyol with a polycarboxylic acid.
U.S. Pat. No. 4,743,672 discloses a sag-resistant, two-component polyurethane adhesive comprising a first polyisocyanate component and a second component comprising a polyol, a poly(alkylene oxide) polyamine, and a polyurethane catalyst. The suggested curing component, B-side, contains low molecular weight (40 to 400 molecular weight) chain extender polyols in amounts ranging from 0 to 30% by weight of the total curative component and small amounts (0 to about 5% by weight) of low molecular weight di- or polyamines (aliphatic or aromatic, primary and secondary amines of molecular weight ranging from 60 to 400) for the purpose of sag-resistant property of the adhesive.
U.S. Pat. No. 5,166,300 discloses an adhesive based on a polyurethane prepolymer which is prepared by reacting a specific combination of polyhydroxy compounds and an isocyanate compound. The polyhydroxy compounds include a polyalkylene ether diol, a polyalkylene ether triol, and a polyester polyol.
U.S. Pat. No 5,606,003 discloses an adhesive containing isocyanate terminated prepolymer component comprising the reaction product of one or more polyols, and one or more polyisocyanates, and a curative component comprising one or more polyol curing agents, having sufficient hydroxyl functionality to form a crosslinked composition when reacted with the isocyanate terminated prepolymer component, and optionally one or more polyamines. The improvement consists of the presence of at least 80 wt. % of one or more polyols used to the isocyanate terminated prepolymer component are one or more grafted poly(alkylene oxide) polyols formed from grafting reactions of poly(alkylene oxide) polyols with acrylonitrile.
Two-component polyurethane adhesives are becoming increasingly useful to adhere structural substrates such as reinforced plastics, metals, wood, and glass. These adhesives can be dispensed either in a gravity flowable state or a non-sag shear-thinning paste system, wherein one component is composed of an isocyanate or isocyanate prepolymer and the second component contains a mixture of a polyhydroxy compounds free of isocyanate groups.
It is known that sag resistance can be improved by incorporating on the curative side containing polyhydroxy components some low molecular weight di-or poly-primary or secondary amine. Thus, the polyurethane adhesive formed by blending the above two components in appropriate mix ratios provides a sag-resistant mixture having non-to-poor initial tack and peel strength, which upon heating or standing at room temperature build the adhesion by curing of polyols with the polyisocyanate.
In boat building, especially glass fiber boats, hulls, decks, stringers and bulkheads are typically made of reinforced fiberglass. The stringer system which is commonly made up of interlocking stringers and bulkheads is assembled outside of the boat, with interlocking pieces being held with staples. Once the system is assembled, it is then placed in a boat as a module and laminated into place using a combination of reinforcing fabrics. Conventional fiber reinforced plastic (FRP) assemblies can be joined mechanically, by adhesives, or by a combination of both. For example, in the assembly of boats, decks are integrally bonded to hulls. One manufacture method involves the so-called glassing-in technique for bonding the deck to the hull. Sheets or pieces of fiberglass fabrics are pre-impregnated with unsaturated polymer resin syrup containing free radical initiators. Glassing-in of the deck-to-hull perimeter is labor intensive and adds additional undesirable weight to the boat. The glassed-in bond is typically rigid and brittle.
Another technique of deck-to-hull assembly is the so-called through-bolting method. A one-component, moisture-curing urethane sealant is first applied around the deck-to-hull perimeter to provide a moisture seal, and bolts are used for structural strength. The sealant does not provide structural strength. It would be industrially important to simplify or eliminate the fastening steps and labor associated with the through-bolt and glassing-in techniques with structural adhesives that could provide both structural integrity and sealing of the bonded surfaces.
Conventional urethane adhesive systems are formulated for relatively rapid curing, e.g. under 15 minutes at ambient temperature in a large bead. The time prior to loss of workability, or wetability of the substrate, by the mixed adhesive is referred to as xe2x80x9copenxe2x80x9d or working time. An open time of about 15 minutes or less under ambient conditions can lead to difficulties for bonding of large parts, especially during warm seasons of the year. Furthermore, in the bonding of large parts, especially those structures having surface irregularities along the mating surfaces, a relatively large bead of adhesive is needed. The exotherm from a reactive two-part urethane particularly applied in a large cross-sectional area bead, e.g. greater than or equal to about 2 cm.xc3x972 cm., in warm ambient conditions, further reduces the open time of conventional adhesives.
It would be industrially advantageous to provide a structural reactive urethane adhesive system that provides a longer open time, that is at least about 25 minutes at 32xc2x0 C. in a large bead without sacrificing cured bond strength while curing under ambient conditions.
In accordance with the invention, a two-part, 100% solids, non-foaming, ambient curable structural urethane adhesive is described which enables the laying down of a large adhesive bead under warm ambient conditions with an open time of at least 25 minutes, more preferably 50 minutes, which enables the manual joining of large structural components with large adhesive beads, and where the assembled structure requires high cured bond strength. The invention resides in a two-part urethane adhesive comprising on the A side a polyisocyanate or isocyanate prepolymer and on the B-side a combination of polyols. The polyols used on both sides comprise at least 75% of polyols containing secondary hydroxyl groups.
The two-part polyurethane structural adhesive according to the invention exhibit at least 25 minutes of open time at 32xc2x0 C. in a large cross-sectional area bead, and cure to high strength under ambient conditions as a composition comprising in one part a isocyanate and/or isocyanate prepolymer made with a secondary polyol, and in a second part, a polyol mixture (I) or (II), wherein 75% to 100% by weight of the polyol mixture (I) or (II) are polyols having secondary hydroxyl functionality and the polyol mixture (I) or (II) contains a long chain polyol defined hereinbelow. The polyol mixture (I) is substantially absent short chain polyols having functionality of 2, and includes
a) an amount from 30% to 80% by weight on total polyol weight in (I) of long chain polyol of molecular weight from 2000 to 12000,
b) and from 20% to 70% by weight on total weight of the polyol mixture of a short chain polyol with functionality greater than 2 and a molecular weight of from 90 to 800; and
mixture (II) comprises
a) an amount from 5% to 80% of the long chain polyol of molecular weight of from 2000 to 12000 together with
b) an amount from 5% to 70% by weight of said mixture of a short chain polyol having a functionality of greater than 2 and a molecular weight of from 90 to 800, and
c) an amount from 5% to 70% of a short chain polyol having a functionality of 2 and a molecular weight of from 90-800.
In another aspect, the invention is directed to a method for joining two substrates, e.g., two structural members, by mixing the first and second parts of the adhesive, applying the mixture to the substrate bonding site, joining the substrate members, and allowing said adhesive to cure under ambient conditions. The invention is particularly adaptable for bonding large composite parts. One such bonding application is joining of a boat deck to stringer. The size of the boat can be between 6 and 30 meters. The adhesive joints can be 2 to 15 cm in diameter and 0.5 to 10 cm in height along the stringer perimeter. The long open time of this invention accommodates the irregularity of the deck-to-stringer bonding gaps. When the two mating surfaces have a tighter tolerance (smaller and even gap in between), the bond-line thickness can be reduced, such as down to 0.8 to 2.5 mm.
The two-part urethane adhesive provides sufficient time to dispense adhesive, position, and align the parts to be joined under ambient conditions, particularly during summer months.